1. Field of the Invention
This invention relates to a table carrier system which drives a table, by measuring a moving amount of the table using an interference of a laser beam.
2. Background Description
Measuring systems used in this kind of tablet carrier system have a mirror mounted on the table and interferometers mounted as a static system. The interferometers emit measuring and reference light beams from one original source. The measuring light beam reflected by the mirror interferes with a reference light. The interferometers detect the interference between the two beams and output 1 pulse per .lambda./n (usually n=4) change of distance between the interferometers and the mirror.
As counts of pulses are multiplied by value of .lambda./n and the product is converted into units of inches or millimeters, the measuring system detects a moving amount of the table.
For example, a laser photo plotter, which has a scanning optical system for scanning the laser beam in a principal scanning direction and a table carrier system for carrying a table in an auxiliary scanning direction normal to the principal scanning direction, uses a method called "scaling" for controlling the table movement.
The process of scaling is used for changing an exposure area of the object such as a printed board from initialization in order to, for example, chemically process the printed board after being exposed. The method of scaling is used to increase or decrease a real moving amount, and not to change a set moving amount value, since a scale of a table driving system is changed otherwise.
Moreover, in order for the scanning optical system to produce scanning lines having equal or uniform pitch, it is necessary to distribute the increasing or decreasing part among all moving amount on an average. Heretofore, that distribution is achieved as a function of a software installed in a control unit for controlling and driving the table.
However, with the afore-mentioned table carrier system, detection of moving amount and control of driving system are executed independently, and a process of scaling is treated as a control of the driving system. Therefore, a minimum unit of the scaling is a unit of the driving system.
Generally, since the resolution of a driving system of a table carrier system is lower than that of position detecting systems, the conventional table carrier system carried out a high accuracy detection, but it could execute only rough accuracy scaling.
This problem can be solved by raising the resolution of the driving system to a value as high as that of the detecting system. However, in this case, a new problem occurs because a load for the driving system becomes larger and/or the software becomes complex.